The overall objective is to determine the thermoregulatory and baroreceptor control of heart rate (HR), gastrointestinal blood flow and leg blood flow in the unanesthetized baboon during heat stress and during the febrile pyrogen reaction (fever). One emphasis will be to measure the HR sensitivity to changes in arterial blood pressure in the normothermic and hyperthermic (via heat stress or pyrogen reaction) baboon to determine if the change in body temperature alters the HR responsiveness to baroreceptor challenges. Arterial blood pressure manipulations will be accomplished via inflation to pneumatic blood vessel occluders chronically implanted around the inferior vena cava and descending aorta. Arterial blood pressure will also be manipulated in normothermic and hyperthermic baboons under sympathetic or vagal blockade induced by pharmacologic agents to determine whether hyperthermia alters the relative sympathetic vs. vagal control of HR during baroreceptor challenges. A second emphasis will be to determine how a febrile pyrogen reaction alters the normal control of hindlimb (cutaneous) blood flow that is exerted by changes in internal body temperature. This study will utilize baboons with chronic femoral arteriovenous shunts into which a heat exchanger will be placed to obtain control of the animal's internal temperature.